1. Field of the Invention
The present invention relates to a coherent modulation method, and particularly to an orthogonally-multiplexed orthogonal amplitude modulation method.
2. Description of the Related Art
Depending on the number of orthogonal multiplexing levels M being used, several modulation formats have been developed for achieving high efficiency in power or in spectrum. The 2N-D biorthogonal modulation that constitutes its 4N-ary signal set by 2N orthogonal signals and the negatives of these signals, disclosed by I. S. Reed et al., in “N-orthgonal phase-modulated codes,”, belongs to the extreme case in which multiplexing is used (i.e., M=1). Typical embodiments are biorthogonal 2NFSK/2PSK with a frequency modulation index μ=½ and biorthogonal NFSK/4PSK with μ=1, which are known to be more efficient both in power and in spectrum than orthogonal 2NFSK with μ=½. In the other extreme case that the maximum number of orthogonal multiplexing levels is used (i.e., M=2N), the 2N-D orthogonally frequency-division-multiplexing (OFDM) biphase modulation is constructed by 2N spectrally-overlapping and orthogonally-multiplexed BPSK-modulated carriers (e.g., 2NOFDM/BPSK with μ=½ and NOFDM/QPSK with μ=1), disclosed by R. W. Chang, “Synthesis of band-limited orthogonal signals for multi-channel data transmission,”. It is noted that NOFDM/QPSK with μ=1 can also be regarded as being constructed by orthogonally-multiplexed QPSK-modulated carriers and thus classified to the case where N levels of orthogonal multiplexing (i.e., M=N) are employed. Due to the use of a large number of orthogonal multiplexing levels, the OFDM signals can provide improved efficiency in spectrum, especially as N is large.
In addition to the above extreme cases, the 2N-D quadrature frequency/phase modulation (NQFPM) was recently developed by S. M. Fleisher et al., in “quadrature frequency/phase modulation,” IEEE Trans. Commun., vol. COM-43, pp. 1513–1524, February/March/April 1995, based on two levels of orthogonal multiplexing (i.e., M=2) NQFPM can be constructed in two equivalent formats. In the multi-frequency format, the NQFPM signal consists of a quadrature-carrier sum of two biorthogonal NFSK/2PSK signals, each with μ=1. In the multi-pulse format, NQFPM is constructed by an orthogonal sum of two N-D biorthogonal component signals, each constituted by the pairwise orthogonal products of N/2 pulses and two quadrature carriers, disclosed by C. D. Chung, “differential detection of quadrature frequency/phase modulation signals,” IEEE Trans. Commun., vol COM-47, pp. 546–557, April 1999. In particular, the multi-pulse 2QFPM signal specializes in the power-efficient quadrature-quadrature phase shift keying (Q2PSK) signal, disclosed by D. Saha and T. G. Birdsall, “quadrature-quadrature phase shift keying,” IEEE Trans. Commun., vol. COM-37, pp. 437–448, May 1989. Since a larger number of orthogonal multiplexing levels is used, NQFPM is more spectrally efficient, but less power-efficient, than biorthogonal 2NFSK/2PSK with μ=½ and biorthogonal NFSK/4PSK with μ=1. On the other, hand, NQFPM is less spectrally efficient, but more power-efficient, than 2NOFDM/BPSK and NOFDM/QPSK, due to the use of a smaller number of orthogonal multiplexing levels.
Although various modulation formats with 1, 2, N and 2N levels of orthogonal multiplexing have been developed, the family of 2N-D modulations employing arbitrary levels of orthogonal multiplexing have not yet exploited.